The present invention relates to a method of and apparatus for aligning a photomask, suitable for use in, for example, a masking exposure step of a selective photo-etching process in the field of the semiconductor industry and, more particularly, to a method of and apparatus for effecting the mutual contact of a semiconductor wafer and a mask, as well as separation of the same from each other.
A mask aligning apparatus has been disclosed, for example, in the specification of U.S. Pat. No. 3,645,622, in which a wafer chuck, supporting a wafer to be aligned with a mask, is disposed beneath the mask. The wafer chuck is capable of up and down movement, so that the wafer may be brought into contact with the lower surface of the mask.
In this type of apparatus, it is essential to press the wafer against the mask at a predetermined contact pressure, without causing damage such as scratching a mask having a certain pattern, e.g., a circuit pattern of a semiconductor integrated circuit, and without damaging the wafer itself. At the same time, it is essential that the wafer and the mask be held in mutual contact, of a predetermined intimacy, during the final exposure step which is effected subsequent to the mask alignment. In the case of a mask having a fine pattern of elements and wirings as those of a large scale integrated circuit, inadequate contact of the mask and the wafer with inferior intimacy will deteriorate the resolution of the fine pattern, resulting in a small yield of the acceptable wafer product.
In the known apparatus of the kind described, no means are provided for finely controlling and adjusting the bend of the mask contacting the wafer during the exposure. Therefore, it is often experienced that the mutual contact of the mask and wafer for the exposure is not made with the desired intimacy in the predetermined plane of contact. If the exposure is effected in such a state, the resolution of the fine pattern will be seriously deteriorated resulting in a reduction of the yield of the acceptable wafer product or a dimensional error of the pattern formed by the exposure.